The invention relates to a process for preparing blends of plastomer type vinylidene fluoride copolymers and fluorosilicone polymers. The blends have improved flex life at low temperature and are useful for lining, coating and jacksting of electrical and optical cables, films and piezelectric items.
It is well known that the polymers and copolymers of vinylidene fluoride of plastomer type are useful materials for lining and coating applications where weather and UV resistance are needed. Their range of applications would be much wider if they exhibited flexibility at temperatures substantially lower than -30.degree. C.
At normal service temperatures, fluoroelastomers such as plastomer type vinylidene fluoride copolymers have higher mechanical properties than fluorosilicone elastomers. However, fluorosilicone elastomers are known for outstanding low temperature properties which are better even than the carbon fluoroelastomers. Also, the fluorosilicone elastomers exhibit better chemical resistance than dimethylsilicone elastomers.
It is also known to try to improve the processability of VDF based gums, in particular the extrudability, by adding a fluorosilicone gum before curing. The curing is effected by using an organic peroxide, and optionally a cocuring agent (or coagent) to obtain a higher cure rate or better compression set. However, the properties at low temperature of the fluorocarbon elastomers in terms of glass transition temperature have not been changed. Experiments carried out by the Applicant have shown that the flexibility at low temperatures of VDF-based plastomers is not improved by adding fluorosilicone elastomers by the method of blending described in the case of VDF-based gums.
It has, surprisingly and unexpectedly, been found that the flexibility at low temperatures of -30.degree. C. or below, of the VDF based copolymers of plastomer type can be improved by using fluorosilicone gums while still maintaining the good mechanical properties of the VDF-type plastomer. The above results are obtained by using the process of mixing the two polymers as described in the present invention.
The advantages of the present invention are obtained by blending a VDF plastomeric copolymers B with a fluorosilicone elastomer A in the presence of a coagent C. However, the VDF plastomeric copolymer is blended with fluorosilicone elastomer A only after the VDF-based plastomer B has been irradiated. The preblending irradiation step was unexpectedly found to be required to produce the improved materials of this invention.
An object of the present invention is therefore to prepare blends of VDF-based plastomer and a fluorosilicone elastomer where the blends are rich in the VDF-based plastomer. The process comprises exposing a composition comprising a VDF-based plastomer B to radiation, mixing a coagent C at a temperature of 80.degree. to 165.degree. C., with the composition comprising a VDF-based plastomer B before or after exposing B to radiation to provide a weight ratio of C/B from 0.05 to 0.4 where coagent C is a polyunsaturated compound capable of grafting to B, forming the blend after the exposure of B to radiation, by mixing 5 to 46 parts by weight of fluorosilicone gum A with 54 to 95 weight parts of the mixture of C and B with the sum of A+B+C being 100 weight parts and A/B equal to 0.05 to 0.9, optionally during the mixing of A with the mixture of B and C, adding a free radical initiator (I.sup.1) characterized by a half life time of about 1 hour at a temperature equal or lower than 95.degree. C., heating the mixture of A, B, C and (I.sup.1) at a temperature of 80.degree. to 140.degree. C., optionally adding one or more additives selected from the group consisting of stabilizers, colorants, fillers, basic-oxides, and lubricants, adding a free radical initiator (I.sup.2) to the blend where the free radical initiator is characterized by a half life time of about 1 hour at a temperature equal or higher than 135.degree. C. and heating to crosslink the blend.
In general in the VDF-based plastomer rich blends, the VDF plastomer B ranges from 54 to 95% by weight and, by weight, is always in higher amounts than the fluorosilicone elastomer A.
The preferred weight ratio of A/B is 0.1-0.45, of C/B is 0.1 to 0.2.
The preferred range of fluorosilicone elastomer A is 10-30% by weight.
By additives it is meant basic oxides, stabilizers, colorants, lubricants, etc. as well known in the field of fluorinated plastomers.
The irradiation is carried out by using high energy radiation, preferably a .gamma.-rays Co.sup.60 source or electron beam. The total dose of a Co.sup.60 source is up to 15 Mrad, preferably in the range 1 to 5 Mrad, at a dose rate of 0.1 Mrad/hour, preferably by exposing to a source with a dose rate of 0.1 Mrad/hour, preferably for 15 hours.
The fluorosilicone elastomer A has the following formula EQU T--O[R.sub.F C.sub.n H.sub.2n Si(CH.sub.3)--O].sub.p [R'(CH.sub.3)SiO].sub.g T (I)
where
g is zero or a number different from zero PA1 p is a number different from zero PA1 R' is methyl, or phenyl, or vinyl, or hydrogen PA1 n is 2 or 3 PA1 R.sub.F is a perfluoralkyl group containing from 1 to 8 carbon atoms PA1 T is H, or --SiR'(CH.sub.3).sub.2
the weight average molecular weight being generally in the range of 100,000 to 800,000, preferably 150,000 to 700,000. The elastomers where R.sub.F is equal to CF.sub.3 or C.sub.4 F.sub.9 are produced by Dow Corning Corporation, U.S.A.
The vinylidene fluoride based plastomer B is:
polyvinylidene fluoride
or a plastomer copolymer vinylydene fluoride-tetrafluoroethylene (VDF-TFE)
or a plastomer copolymer vinylidene fluoride-hexafluoropropene (VDF-HFP)
or a plastomer copolymer vinylidene fluoride-tetrafluoroethylene-hexafluoropropene (VDF-TFE-HFP).
The plastomers are commercially available from Atochem North America, subsidiary of Atochem France, the weight average molecular weight being generally in the range of 100,000 to 800,000, preferably 130,000 to 600,000.
The plastomers, as it is well known, show a certain degree of crystallinity and have therefore a defined melting point range. Generally the homopolymers have a melting temperature ranging from 156.degree. to 170.degree. C., the copolymers VDF/TFE from 122.degree. to 126.degree. C., the copolymers VDF/HFP from 140.degree. to 145.degree. C.
The reactive coagent C is preferably selected from diallyl phthalate (DAP) or triallylisocyanurate (TAIC) both available from Aldrich Chemical or their mixtures.
More preferably the component C was a mixture of DAP and TAIC in a weight ratio from 1/3 to 3.
The blend of B with C and A is effected for example in a Brabender Plastograph mixer PL 35 having the jacket at controlled temperatures for example in the range of 130.degree.-165.degree. C. The blending was effected preferably at about 140.degree. C.
When the radical initiator I.sup.1 is present, the mixing time is generally 1-15 minutes, depending on the temperature generally ranging from 80.degree. to 140.degree. C. I.sup.1 radical initiators are for example peroxide such as benzoylperoxide and lauroylperoxide. The weight ratio of I.sup.1 to the plastomer B is in the range 2.times.10.sup.-3 to 4.times.10.sup.-2.
The I.sup.2 radical type of initiators are characterized by having a one hour half life at a temperature of at least 135.degree. C. Preferably the I.sup.2 initiators are selected from dicumyl peroxide; di-t-butyl peroxide; 2,5-bis(t-butylperoxy)-2,5-dimethylhexyne-3 (LUPERSOL.RTM.130), 2,2-bis(t-butylperoxy)-1,4-diisopropylbenzene. The weight ratio of the initiator (I.sup.2) to the blend of polymers was in the range of 0.01 to 0.04 weight part for 100 parts by weight of A+B+C.